1. Field of the Invention
This invention relates to a method for reducing a visible NO2 plume in an exhaust stack of a combustion or xe2x80x9cgasxe2x80x9d turbine power plant, other power plants or other combustion systems, including those in stationary or mobile applications.
2. Background Information
Combustion turbines used in power plants comprise a compressor section, a combustion section, and a turbine section. A supply of air is compressed in the compressor section and directed into the combustion section. Fuel enters the combustion section by means of a nozzle. The compressed air enters the combustion inlet and is mixed with the fuel, which is then combusted to produce a high-temperature, high-pressure gas. This gas then exits past the combustor section via a transition section and is injected into the turbine section to run the turbine.
The hot gases produced by the combustion section exit the power turbine and pass into a duct designed to convey these gases either to a heat exchanger (where additional energy is extracted) or directly to the exhaust stack. These exhaust gases will range in temperature from 600xc2x0 F. to 1200xc2x0 F. depending upon the type of fuel, the load on the engine, and the ambient temperature.
As a result of combustion, NOx (a mixture of NO and NO2) is formed. The NOx produced by the gas turbine, or any subsequent post-combustion firing, is released into the atmosphere unless some control mechanism is used. The NO2 component is a strong colorant in exhaust gases, and absorbs light in the blue region, including light in the wavelength between about 350-400 nm, resulting in a plume that appears from yellowish to orange-red.
The NO2 plume is evident in many DLN (dry low NOx) combustion systems, and determination of the actual source and mechanism of formation of NO2 in DLN systems has proven to be an intractable problem, limiting options for controlling it. NO2 may even be produced in conventional combustion turbine systems and also from some chemical processing facilities, such as nitric acid production plants. Control of NO2 is important because, among other reasons, concentrations of only 20-30 ppm can produce a very objectionable plume.
NO2 plumes may also be present in other combustion systems different from the gas turbine. These include pulverized coal (pc), fired boilers (operating according to the Rankine cycle), diesel engines, and gasoline engines (operating according to the Otto cycle). In particular, coal/oil energy systems and diesel cycle energy conversion systems produce significant NOx levels and may have a noticeable plume.
In gas turbine applications, NOx emissions are most often reduced using emission control systems which employ chemical means, such as ammonia injection in an SCR (selective catalytic reduction) system. Photometric methods have been studied in coal combustion where NOx is found in combination with other pollutants such as sulfur dioxide (SO2). SO2 and NOx are combustion products related to fuel burning associated with heat or power production. Photometric remediation methods include illumination or irradiation of the exhaust gases prior to subsequent processing steps, but all require additional steps or processing parameters during the reduction process in order for NO2 to be reduced. Many of the photometric methods reviewed result in formation of undesirable byproducts such as sulfuric acid mist, ozone, and particulates.
For example, U.S. Pat. No. 4,995,955 (Kim et al.) discloses a process for reducing NOx contamination within an effluent stream using an ultraviolet light source having a wavelength of less than 220 nm. This method requires particle filtration or electrostatic precipitators to remove particles formed from the chemical processes used to clean the exhaust gases. Using a wavelength in this region causes the formation of ozone, a well-known component of urban smog.
U.S. Pat. No. 3,869,362 (Machi et al.) discloses a process for reducing NOx/SO2 emissions by controlling the ratio of NOx to SO2 before introducing the gas mixture into an irradiation chamber. This method requires the presence of SO2 and also additionally requires collecting chambers to cleanse the exhaust stream of mist and solid particles.
U.S. Pat. No. 3,984,296 (Richards) discloses a photochemical process for removing gaseous pollutant compounds (including NOx) from a contaminant gas stream, by introducing positive and/or negative ions into the flue gas prior to irradiation. The ions form complexes with the pollutants which must be removed by precipitation or other methods.
U.S. Pat. No. 4,146,450 (Araki et al.) discloses a method for reducing or removing NO2 from exhaust gases containing NOx using a catalytic reduction method with ammonia which has been previously excited by ultraviolet radiation.
It is desirable, therefore, to provide a method of exhaust gas NO2 decomposition without the need for additional processing requirements such as additional chemicals, temperature control, particle collection of precipitates, or other processing steps.
Accordingly, the present invention provides a method of reducing a visible NO2 plume in an exhaust stack by irradiating the exhaust gases within the duct or stack with at least one illumination source. The visible plume is substantially reduced while minimizing the formation of additional pollutants. This reduction may result in NO2 concentrations of below 20-30 ppm, and may represent an NO2 reduction of 90% or more in some cases. The reduction process is effective irrespective of the source of the NO2 (including the combustion of fuel gas, oil, or solid fuel). The degree of reduction will be a function of, among other things, the initial NO2 concentration, lamp intensity, and path length.
The illumination source can be placed directly within the exhaust duct/stack or optically coupled to these, and has a wavelength of light of at least 350 nm and preferably between 350 and 400 nm. The method does not require the presence of other chemicals or additional processing steps to achieve its desired reduction.
It is an object of the invention therefore to provide a method of reducing a visible NO2 plume within the exhaust.
It is a further object of the invention to provide a method of reducing a visible NO2 plume in exhaust gasses by irradiating the exhaust stack gasses with a source of illumination within the stack or optically coupled thereto.
It is an additional object of the invention to provide such a method of reducing a visible NO2 plume without requiring the presence of other chemicals or other processing steps to achieve the desired reduction.
It is a further object of the invention to reduce a visible NO2 plume in exhaust without producing undesirable by-products.
These and other objects of the invention will be apparent from the following description and appended claims.